1. Field of the Invention
The present invention is directed to a point interferometer to measure phase shifts in reticles, and more particularly, to a common path image shearing interferometer that provides two point images with adjustable separation therebetween.
2. Discussion of the Prior Art
FIG. 1 shows a reticle 10 which is used during the manufacture of integrated circuits. For example, reticles are used to pattern semiconductor material, which is later etched and processed to form various regions therein. Various types of lithography are used to pattern and etch semiconductor material, such as photo-lithography and phase shift lithography. During phase shift lithography, a reticle is used as a mask over the semiconductor material.
The reticle causes a phase shift of light passing therethrough. In particular, incident light 12 enters an entrance surface 14 of the reticle 10, passes therethrough, and emerges from an exit surface 16 as an exit light 18. Due to the thickness 20 of the reticle 10, i.e., the distance traveled by the light within the reticle 10, the phase of the incident light 12 is different from the phase of the exit light 18.
For use in lithography, the entrance surface 14 of the reticle 10 is patterned to have depressions or steps, referred to as phase steps 22. The reduced thickness 24 of the reticle 10 below the phase step 22 provides a phase shift of the incident light which is different from the phase shift caused by the thicker portion 20 of the reticle 10.
In particular, light 26 incident on the phase step 22 emerges from the reticle as a step exit light 28. The light 26 incident on the phase step 22 has the same phase as the light 12 incident on the reticule's main or entrance surface 14. However, due to the difference in the distances traveled within the reticle 10, the two exit lights 18, 28 have different phases. Thus, different portions of the exit light have different phase shifts, depending on the distance traveled in the reticle; light 26 entering the phase step 22 travels a shorter distance 24 in the reticle 10, than the distance 20 traveled by light 12 entering the upper or main entrance surface 14 of the reticle 10.
Manufacturing smaller semiconductor devices, requires a decrease in the areas of different phase steps of a reticle. The decreased areas and increased density of adjacent phase steps present difficulties in measuring the phase steps on the reticle. For example, an area of interest on the reticle may be less than a micron. To spatially resolve such small areas requires an optical interferometer operating at a high numerical aperture (NA).
Further, the wavelengths at which the measurement must be made are also decreasing, such as 190 nm. The small phase step areas and short wavelengths limit the choice of lenses and interferometers. This is because the lenses and interferometers must simultaneously allow deep ultra-violet (DUV) transmission and have a high numerical aperture (NA). Conventional interferometers cannot properly measure phase shifts when the phase shift areas or steps of a reticle are small and close together. Rather, conventional interferometers are used to measure the phase on the reticle using relatively large areas.